CORN IS GROWN ON MORE ACRES OF IOWA LAND THAN ANY OTHER CROP.
Planted acreage reached a high in 1981 with 14.4 million acres planted for all purposes
and has hovered near 12.5 million acres since the early 1990s. Table 1 shows the trend in
corn acreage, yield, and production since 1900 as reported by the Iowa Agricultural
Statistics Office. Total production and average yield per acre records were set during the
1994 season. • Corn yields have trended upward over the past 40 years. Considerable
year-to-year variation indicates that the weather has a major influence on corn
production. Many factors are responsible for these yield increases, including better hybrids,
more efficient use of fertilizers, better pest management (weeds, insects, and diseases),
and better management overall. Producer-controlled management factors (row spacing,
planting dates, planting depth, plant densities, and replant options) that affect yield will
be addressed in this guide.
PM 1885
September 2001
Planting Dates and Planting Depth
year to year; however, finishing planting
by mid-May is a goal most producers
should strive to achieve . . . More recent
studies indicate that the optimum time
for planting corn in Iowa still falls
between April 20 and May 5.
I
FIGURE 1
until late May or early June for several
reasons: 1) time was needed for spring
plowing; 2) soils were warmer at later
dates, contributing to the success of
less vigorous races or hybrids; and 3)
preplant tillage facilitated weed control. As more vigorous hybrids came
into use along with herbicides for weed
control, planting dates for corn were
moved earlier and earlier. Studies in
the 1980s in Iowa indicated that on
average, the best time to plant corn was
from April 20 to May 10.
The optimum time to plant varies
from year to year; however, finishing
planting by mid-May is a goal most
producers should strive to achieve
Effect of planting date on corn grain yield
response in Iowa (1997–2000).
Relative yield (%)
The optimum time to plant varies from
IN THE PAST, CORN PLANTING WAS DELAYED
100
90
80
70
60
50
Apr 10 Apr 24 May 8 May 22 Jun 5 Jun 19
Average planting time
(Fig. 1). Corn can be planted before
the threat of freezing temperatures has
passed for two reasons: 1) the time
delay between planting and emergence, and 2) the growing point
TABLE 1
Iowa corn grain harvested acreage, average yield, and total production (by
decade) from 1900 to 2000.
A 2-inch planting depth is ideal for
average conditions.
Year
Harvested acres
(x 1,000)
Average yield per acre
(bushels)
Production
(x 1,000 bushels)
1900s
1910s
1920s
1930s
1940s
1950s
1960s
1970s
1980s
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
9,378
9,774
9,267
9,220
9,884
10,184
10,274
11,958
12,045
12,400
12,200
12.950
11,000
12,600
11,600
12,400
11,900
12,200
11,800
12,000
38
39
40
38
51
56
83
100
115
126
117
147
80
152
123
138
138
145
149
145
356,922
379,487
373,250
357,555
505,706
569,737
843,417
1,197,463
1,395,875
1,562,400
1,427,400
1,903,650
880,000
1,915,200
1,426,800
1,711,200
1,642,200
1,769,000
1,758,200
1,740,000
Source: Iowa Agricultural Statistics Office
2
remains below the soil surface for
some time following emergence. More
recent studies indicate that the optimum time for planting corn in Iowa
still falls between April 20 and May 5
(Table 2). However, numerous factors
influence the decision of when to start
planting corn. Rather than an exact
date, soil condition is a key factor to
consider. Planting when soils are too
wet is not advised, regardless of the
date. Under most circumstances, the
best time to begin planting corn in
Iowa is as early as conditions (soil and
temperature) allow.
Many producers use soil temperature as a guide to start planting.
Planting corn in April generally is not
advisable until the soil temperature at
planting depth is near 50° F or above.
Time to emergence in the field ranges
from greater than three weeks for 50 to
55° F soil temperatures to less than one
week for soil temperatures greater than
70° F. Soil temperature can be ignored
if planting is in May; the condition of
the seedbed becomes the critical
issue then.
How early one needs to begin planting corn in order to be completed by
mid-May depends on soil conditions,
acres planted per day, work days available, and total number of acres to be
planted. During late April and early
May, only about half the days normally are suited for field work. A useful
method for estimating a start date for
corn planting is to add up the number
of days expected to plant all acres plus
the number of days of anticipated
weather, mechanical, and personal
delays, and back up that many days
TABLE 2
Corn yield response to planting date.*
Date
April 20–May 5
May 13–19
May 26–June 1
June 10–16
June 24–28
Relative yield potential
(percent)
100
99
90
68
52
* Average of three locations (Nashua, Ames, and Lewis) and three years (1998-2000).
from May 10. That date should be a
target date for beginning corn planting. From a practical standpoint, April
15 is about the earliest planting date
most farmers should consider.
Corn yields do not begin to decline
rapidly until planting is delayed beyond
mid-May (Fig. 1). Corn hybrids used
today appear to have more yield stability over time in the early part of the
season. If planting is delayed until late
May, hybrids characterized as earlier
than “full season” should be used, especially in northern Iowa. A rule of
thumb for changing hybrid maturities
is if planting is delayed until May 25,
select a hybrid that matures five days
earlier than an adapted full season
hybrid for that area. If planting is
delayed another seven to ten days
beyond that, select a hybrid that
matures another five days earlier than
the previous one. Generally speaking,
if corn planting is delayed until June
10 to 15, very early maturing corn
hybrids should be used; after June 10
to 15, corn planting is risky unless the
corn can be used for silage.
Ideal planting depth varies with soil
and weather conditions. Generally
speaking, a 2-inch depth is ideal for
average conditions. When planting
early (April) and soils are cool, the ideal
depth may be slightly less. Planting
depth should never be shallower than
1.5 inches, however. When the surface
soil is dry, especially when planting has
been delayed until mid- or late May,
planting depth may need to be
increased to 2 inches or more. Careful
control of planting depth will improve
stand levels and produce more even
plant emergence.
3
Plant Population
P
PLANT POPULATION REFERS TO THE NUMBER
Optimum plant population depends on
factors such as hybrid, moisture stress
level, soil fertility, and yield goal.
FIGURE 2
Relative yield (%)
Effect of plant population on corn grain
yield response in Iowa (1997–2000).
100
90
80
70
60
15,000 20,000 25,000 30,000 35,000 40,000 45,000
Average planting time
of plants per acre; planting or seeding
rate refers to the number of seeds
planted per acre. The two should not
be confused. Producers typically need
to plant more seed than their intended
plant population goal. How much
more will depend on the germination
percentage of the seed being used, soil
conditions, and anticipated pest problems. Tables 3 and 4 should be helpful
in making planting rate and plant population measurements.
Average plant populations for the
state of Iowa have shown a steady
increase over the last decade. Plant
populations have increased at a rate of
400 plants per acre since 1992. Much
of this increase can be attributed to
improved hybrids that are much more
tolerant of high population stress than
hybrids used previously. Figure 2
shows the response of plant population
on corn grain yields.
Optimum plant population depends
on factors such as hybrid, moisture
stress level, soil fertility, and yield goal.
Each producer should adjust populations according to these factors. The
optimum plant population will not
only vary between regions of the state,
but from season to season, and field to
field on the same farm. Research
results from Iowa State University are
presented in Table 5. These data show
that optimum populations do vary
across locations, but these variations
are relatively small. When environmental conditions are favorable, soil
fertility levels are optimized, and
appropriate hybrids are selected, optimum yields likely will occur when
populations in the range of 28,000 to
32,000 plants per acre are used.
Recommendations for silage production are 2,000 to 4,000 plants per acre
higher than for grain production.
TABLE 3
TABLE 4
Relationship between seeds planted, distance between seeds, and
projected final stand.
Method of estimating
plant population.*
Seeds planted
(per acre)
15
24,000
26,000
28,000
30,000
32,000
34,000
36,000
38,000
40,000
42,000
44,000
17.4
16.1
14.9
13.9
13.1
12.3
11.9
11.0
10.5
10.0
9.5
* Assuming 15 percent mortality.
4
Row spacing, inches
20
30
36
(inches between seeds)
13.1
12.1
11.2
10.5
9.8
9.2
8.7
8.3
7.8
7.5
7.1
8.7
8.0
7.5
7.0
6.5
6.1
5.8
5.5
5.2
5.0
4.8
7.3
6.7
6.2
5.8
5.4
5.1
4.8
4.6
4.4
4.1
4.0
38
Plant population*
(per acre)
Row width
(inches)
Length of row
(in feet equal to 1⁄1000th acre)
6.9
6.3
5.9
5.5
5.2
4.9
4.6
4.3
4.1
3.9
3.8
20,400
22,100
23,800
25,500
27,200
28,900
30,600
32,300
34,000
35,700
37,400
15
20
30
36
38
34' 10"
26' 2"
17' 5"
14' 6"
13' 9"
* Count plants (not tillers) and multiply by
1,000 to estimate plants per acre.
Row Spacing
C
CORN ROW SPACING HAS BEEN STUDIED
for decades with very mixed results. A
2000 survey of Iowa producers reported that approximately 56 percent of
the corn acres in Iowa were in 30-inch
rows with approximately 43 percent of
the corn acres in row spacings greater
than 30 inches and 1 percent in row
spacings less than 30 inches. The trend
toward narrower row spacings continues, but identifying the optimum
row spacing for maximizing corn
production is still a challenge for many
producers.
Recent research at Iowa State
University has examined the effect of
15-, 30- and 38-inch row spacings on
corn grain yields. The results of these
studies are summarized in Table 6.
Narrowing rows to less than 30 inches
showed very small and inconsistent
results with the overall effect showing
no yield benefit for row spacings less
than 30 inches. Previous research
comparing 30- and 38-inch row
spacings indicated a 4 to 6 percent
yield advantage for 30-inch rows. More
recent research, however, suggests that
the yield benefit will vary from year to
year but the overall effect has decreased
to only a 3 percent advantage for
30-inch rows.
30-inch row spacing (left) versus 15-inch
row spacing (right).
TABLE 5
TABLE 6
Corn yield response to plant population at six Iowa locations, 1997-2000.
Relative yield differences of corn
grown in 15- and 38-inch row
spacings when compared with
30-inch row spacings.
Location
Northwest North Northeast Central Southwest Southeast State
Central
Average
Final Years
Relative yield potential
stand tested
(percent)
44,000
36,000
32,000
28,000
24,000
20,000
1
4
4
4
4
1
90
92
100
90
95
78
90
95
100
95
95
79
93
97
100
92
92
76
99
100
99
96
92
89
86
93
100
90
93
78
—
98
98
100
97
—
93
97
100
94
94
81
Relative yield difference (percent)
compared with 30-inch rows
Year
15-inch
38-inch
1995
1996
1997
1998
1999
2000
Average
+2.2
0
-1.9
+1.2
-1.2
+1.8
+0.3
—
—
—
-7.5
-0.5
-2.7
-2.9
5
Replanting Decisions
G
GENERALLY, A CONSIDERABLE AMOUNT OF
Interseeding into an existing thin
stand can be risky.
To be profitable, a replant yield must
cover the extra costs involved with
replanting. This may include the costs of
tillage, planting, seed, and any additional
fertilizers and pesticides if needed.
stress accompanies a decision to
replant. Reasons for replanting are
numerous and the decision to replant
presents a whole series of problems
that producers must consider. Yields
likely will be lower and grain moisture
higher at harvest. Earlier maturing
hybrids likely will be required, or
perhaps switching to an alternate crop
such as soybeans will be necessary.
If soybeans becomes a better choice,
will there be herbicide problems?
Many difficult decisions must be made
rather quickly.
There are some preliminary considerations that all producers must take
into account before making the decision to replant. The first should be to
thoroughly evaluate the current stand,
both in terms of the plant population,
the uniformity of the stand, and the
overall health of the existing plants.
Stand counting was covered previously
(see Table 4). Take enough counts so
that a representative view of the
replant area is achieved.
Table 7 combines the plant population and planting date information for
corn into a single table to aid in
making replant decisions. It is assumed
that hybrids of an optimum maturity
for each date are used. Yields are based
on an even plant distribution, which is
often not the case in a replant situation. A University of Illinois study
indicates yields are reduced by about 2
percent if the stand includes small gaps
of 1 to 3 feet. Numerous gaps of 4 to 6
feet in length may reduce yields by an
additional 5 to 6 percent.
Following is an example of how to
use Table 7:
1. Corn was planted on April 25
near Ames, Iowa, with the desired
population of 32,000 plants per acre.
2. In late May, it was observed that only
16,000 plants per acre were present
and the stand was somewhat uneven.
3. It would be possible to replant on
June 1. Should the field be replanted?
Based on Table 7, the April 25
planting date and 32,000 population
goal would produce 100 percent of full
yield potential. A stand of 16,000
plants per acre planted on the same
date would produce 74 percent of full
yield potential, minus an additional 2
percent for some unevenness, or 72
percent yield potential. If the field were
replanted with a target population of
TABLE 7
Influence of planting date and plant population on corn grain yields.
Final Stand*
28,000–32,000
24,000
20,000
16,000
12,000
Planting Date
April 20–May 5 May 13–19 May 26–June 1 June 10–16 June 24–28
Relative yield potential (percent)
100
94
81
74
68
* Assumes a uniform plant spacing.
6
99
93
80
73
67
90
85
73
67
61
68
64
55
50
46
52
49
42
38
35
result in reduced yields. Plant population of the existing stand should be
noted and the replant planting rate
should be adjusted to account for these
plants, thus ensuring that final plant
populations are not excessive. In addition, the planter operator must drive
carefully so that minimal damage is
done to the existing plants.
Crop maturity choices relating to
planting date have been discussed and
the same guidelines should be followed
for replant situations. If replanting is
considered late enough in the season,
choice of an alternate crop such as soybeans may be warranted. This decision
should be based on the economic
potential of both crops for a given
planting date and should take into
account the yield potential of both
crops for that planting date, the costs
associated with replanting, as well as
the expected market price. As planting
date is delayed, the yield potentials for
corn and soybeans decline at fairly similar rates (see ISU Extension publication PM 1851, Soybean Replant
Decisions, for information on soybean
replanting). Under most circumstances, however, the relative advantage
will move rather strongly towards soybeans after June 10 to 15. Figure 3 can
be used as a guide for determining
whether corn or soybeans should be
selected for planting based on yield
potentials and market prices. In
essence, if corn yields are three times
greater than soybean yields (150
bu/acre vs. 50 bu/acre), soybean market price must be three times greater
than corn or more in order for soybeans to be more profitable to plant
than corn. If corn yields are four times
greater than soybean yields, soybean
price must be four times greater than
corn, and so on. The relationship is linear and does not take into account the
cost of production for replanting either
FIGURE 3
Relationship between the ratio of corn:
soybean yield and the ratio of soybean:
corn price. Figures are based on gross
revenue and do not take into account
any costs of production for either crop.
6
Soybean: Corn price ratio
32,000 plants per acre on June 1, the
yield potential in this case would be 90
percent of full potential. Replanting on
June 1 would produce an 18 percent
(90 percent minus 72 percent) yield
increase over leaving the existing stand.
For a yield potential of 160 bushels per
acre, this would mean an increase of
approximately 29 bushels per acre.
These figures also assume that the
stand achieved by replanting is 32,000
plants per acre. If the original stand
was poor because of disease, insect, or
herbicide injury, it is highly possible
that the replant stand could be diminished also, resulting in a lower return
on the replant investment. The information in Table 7 should not be considered “absolute fact,” but merely as
guidelines for helping make replanting
decisions. Each case will be somewhat
unique and must be considered on its
own merits.
Other replanting considerations
include the costs of replanting, crop
maturity choices, or alternate crop
choices. To be profitable, a replant
yield must cover the extra costs
involved with replanting. This may
include the costs of tillage, planting,
seed, and any additional fertilizers and
pesticides if needed. Replanting costs
may amount to $30 to $50 per acre or
more, depending on the situation. In
some cases, interseeding into the existing stand (drilling seed alongside the
existing rows of plants) may save considerable time and expense related to
replanting. This should be done with
caution, however, and only after a
thorough evaluation of the existing
stand is made noting the current stand
level and stage or size of the plants. To
increase the success of this type of
replanting, crop stage should be V2
(two leaves with collars exposed) or
smaller. Interseeding into stands that
are larger than V2 is risky and may
5
Advantage soybean
4
3
Advantage corn
2
1
1
2
3
4
5
6
crop or the expenditures already realized for the initial planting.
Following is an example of how to
use Fig. 3:
1. A producer is faced with a replant
decision and is wondering whether
to plant corn or soybeans.
2. The producer determines that corn
yields on average are 3.5 times greater
than soybean yields (taking into
account the yield penalties for the
delayed planting).
3. The producer also determines that
expected soybean prices will be
approximately 5 times greater than
corn prices.
Based on Fig. 3, replanting to soybeans would be the obvious choice.
There are, however, several other less
obvious factors that may influence
crop choice for replanting. Herbicide
program, fertilizer program, desired
rotation, potential crop pest problems,
livestock feed needs, and erosion concerns on sloping soils all should be
considered before making the decision
to replant to an alternate crop. When
considering all these other factors,
replanting to soybeans may be the
obvious choice, but it wouldn’t
necessarily be the most practical or
profitable choice.
7
Summary
T
THERE ARE NUMEROUS CROP MANAGEMENT
Recent research at Iowa State University
shows little yield advantage to narrowing
row spacings less than 30 inches. There
is a small (3 percent) yield advantage for
corn grown in 30-inch rows compared
with corn grown in 38-inch rows.
factors that corn producers can use to
their advantage in order to optimize
yields. These include planting date,
planting depth, plant population, row
spacing, and replanting decisions.
Understanding the impact each of
these factors has on corn yield, as well
as any interactions they have together
should enable producers to optimize
their corn production systems.
Early planting should be encouraged, but close attention must be paid
to weather and soil conditions. The
target window should be April 20 to
May 5. Corn yields will decline slowly
as planting is delayed until mid-May
but then begin a fairly rapid decline as
planting is delayed until late May or
later. Optimum planting depth is
dependent on weather and soil conditions also. Ideal depth is 2 inches, but
may be adjusted either shallower or
deeper if soils are cool or dry.
Minimum planting depth should be
1.5 inches regardless of conditions.
Planting rate and plant population
are not synonymous terms. Plant population is the number of plants per
acre. Under optimum conditions,
plant populations in the range of
Prepared by Dale Farnham, Department of
Agronomy, Iowa State University.
Edited by Del Marks, Extension
Communication Systems.
Designed by Mary Sailer, Spring Valley Studio.
8
28,000 to 32,000 plants per acre
should maximize yields. The optimum
plant population will vary with year,
location, soil and weather conditions,
and yield goal.
A majority of the corn acres in Iowa
are planted in rows spaced 30 inches
apart. There is, however, an appreciable acreage planted in rows spaced
wider than 30 inches and a very small
percentage planted in rows narrower
than 30 inches. Recent research at
Iowa State University shows little yield
advantage to narrowing row spacings
less than 30 inches. There is a small
(3 percent) yield advantage for corn
grown in 30-inch rows compared with
corn grown in 38-inch rows.
A replant decision is one of the
most difficult decisions a producer
must make. Many factors are involved
in the decision, including a thorough
evaluation of the original stand, and
each factor should be considered carefully before a final decision is made. If
an alternate crop is selected, consideration should be given to the current
herbicide program, fertilizer program,
desired rotation, potential crop pest
problems, livestock feed needs, and
erosion concerns before making the
decision to replant to the alternate crop.
File: Agronomy 2-2
[C]
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The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color,
national origin, gender, religion, age, disability, political beliefs,
sexual orientation, and marital or family status. (Not all prohibited
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Issued in furtherance of Cooperative Extension work, Acts of
May 8 and June 30, 1914, in cooperation with the U.S.
Department of Agriculture. Stanley R. Johnson, director,
Cooperative Extension Service, Iowa State University of Science and
Technology, Ames, Iowa.